The present invention relates generally to filter system for rapidly removing arsenic from contaminated drinking water. More specifically, the present invention relates to a hybrid filter bed system that significantly improves arsenic removal form drinking water using zero-valent iron particles.
Arsenic is a toxic element that naturally occurs in a variety of combined forms in the earth. Its presence in natural waters may originate, for example, from geochemical reactions, industrial waste discharges and past agricultural uses of arsenic-containing pesticides. Because the presence of high levels of arsenic may have carcinogenic and other deleterious effects on living organisms, the U.S. Environmental Protection Agency (EPA) and the World Health Organization have set the maximum contaminant level (MCL) for arsenic in drinking water at 10 parts per billion (ppb). Further, there are regulatory proposals that the maximum arsenic levels be no more than 2 ppb.
Water produced by many municipal water systems, particularly in the western United States as well as other locals, typically has arsenic levels up to about 50 ppb and higher and thus is commonly higher than proposed lower levels for arsenic, and is in some instances higher than currently accepted levels for arsenic. Some purification means must be employed to remove arsenic prior to consumption of the water. The problem is compounded by the presence of minerals, including carbonates, which interfere with many purification schemes and methods. Particularly with water obtained from areas with geologic evidence of volcanic activity, both high arsenic levels and high mineral content, including carbonates, are typical.
Various technologies have been used in the past to remove arsenic from aqueous systems. Examples of such techniques include adsorption on high surface area materials, such as alumina and activated carbon, ion exchange with anion exchange resins, co-precipitation and electro-dialysis. However, most technologies for arsenic removal are hindered by the difficulty of removing the arsenite form of the contamination.
It is known that the oxidation rate at the ZVI-water interface determines the nature of the oxides and oxyhydroxides formed on the iron surface, as well as the final corrosion products. These corrosion products include lepidocrocite and magnetite with green rust and bernalite as intermediates. Studies of the geometry of co-precipitated and adsorbed arsenate on the ZVI corrosion products ferrihydrite, goethite, akageneite, and lepidocrocite, concluded that arsenate ligands primarily form an inner sphere bidentate arsenate complex. Monodentate arsenate complexes have also been observed. Once arsenic anions have been “fixed” on active sites in the iron corrosion products, they are unlikely to desorb. Studies have found that only about 7-11% of adsorbed arsenic was released from exposed iron filings when flushed with 0.01 M NaNO3 in DI water. Additionally, as the arsenic complexes age, the rate of desorption decreases significantly, attaining 5.01-5.72% after 30 days and 2.88-4.29% after 60 days.
The more successful techniques that have been used in large municipal water supplies are not practical for smaller applications because of space requirements and the need to use dangerous chemicals. The two most common techniques for smaller water treatment systems have been reverse osmosis and activated alumina. The former method produces arsenic-containing waste streams that must be disposed of and the latter requires the use of caustic chemicals.
While many of these processes provide acceptable results only within narrow and restrictive parameters, most of these processes are costly and comparatively inefficient.
Further, none of the prior art methods meet the requirements of efficient removal of arsenic utilizing commonly and inexpensively available reagents with a minimum of mechanical processing and steps. Thus there is a need for an inexpensive and simple process that specifically removes arsenic from drinking water, such as municipal water systems and rural well systems.